Horizontal cell potentials: dependence on external sodium ion concentration.

The membrane potential of the horizontal cell of the axolotl is highly dependent on the extracellular concentration of sodium. Experimental results reported here are consistent with the suggestion that in the dark the receptors release a synaptic transmitter which increases primarily the sodium conductance of the postsynaptic membrane. Externally applied aspartate or glutamate depolarizes the horizontal cell membrane and eliminates the light response of the horizontal cell. However, it appears to increase the conductances of the postsynaptic membrane to several ions rather than just to sodium ions.